Reduction of low frequency regeneration and hum in multistage audio amplifiers



Dec. 5, 1944. F. E. TERMAN 2,364,403

REDUCTION OF LOW FREQUENCY REGENERATION AND HUM IN MULTISTAGE AUDIOAMPLIFIERS Filed Feb. 10, 1942 2 Sheets-Sheet l Va; M615 0VLOP0 4cm;mvpzpxwczz INVENTOR ATTORNEY Dec. 5, 1944. F. E. TERMAN 2,364,403

REDUCTION OF LOW FREQUENCY REGENERATION AND HUM IN MUL'IISTAGE AUDIOAMPLIFIERS Filed Feb. 10, 1942 2 Sheets-Sheet 2 [@U/I/AL E/VT CIRCUIT OFNORMAL REGE/VfR/l 770 Fyl fQU/VALE/VT C/RCU/T 0F COMPENSAT/NG VOLTAGEINVENTOR [75113977714240 ATTORNEY Patented Dec. 1944 REDUCTION OF LOWFREQUENCY REGEN- ERATION AND HUM AUDIO AMPLIFIERS Frederick E. Terman,Stanford University, Calif.,

assignor to International Standard Electric Corporation, New York, N.Y., a corporation Delaware In MULTISTAGE Application February 10, 1942,Serial No. 430,202

.6 Claims.

This invention relates to new and useful improvements in audio-frequencyamplifiers.

The object of the invention is to provide a simple and effective systemto control w froquency regeneration by balancing the feedback to theplate circuit of a low level amplifier.

Regeneration at low frequencies arising from a plate impedance common toseveral stages is one of the most troublesome problems encoun-- tered inaudio-frequency amplifiers. Such rcgeneration arises from the fact thatamplified currents in the final stage of the amplifier will produce avoltage across the internal impedance of the power-supply system. Ifstages of lower power level use the same power supply system, thisvoltage developed across the internal impedance will then be inserted inthe plate circuits of these low-level stages. The result is regenerationthat causes either a modification of the amplification characteristics,or low frequency oscillations (motor-boating). Troubles of this sort areespecially severe in the case of high-gain amplifiers having a good lowfrequency response.

The usual remedies for regeneration arising from a common plateimpedance are to make the low frequency response of the amplifier poorand to employ decoupling filters. If these fail, separate power suppliesare used for the high and low level parts of the amplifier.

In accordance with the present invention the feedback to the platecircuit of a low-level amplifier stage is balanced by an equal andopposite feedback developed across the cathode biasing resistor of thesucceeding amplifier stage. This balancing system can be applied inaddition to other means normally used to control low-frequencyregeneration, and when properly adjusted will give a very greatreduction in the net feedback.

In the drawings Fig. l diagrammatically illustrates those elements of anembodiment of the invention which are necessary for a clearunderstanding thereof;

Fig. 2 an equivalent circuit of normal regeneration; and

Fig. 3 an equivalent circuit of the compensating voltage developedaccording to the present invention.

Fig. 1 shows three stages of a multl-stage audio frequency amplifier.T1, T2 and T! are amplifying tubes in the first, second and final stagesof the amplifier. Additional intermediate stages may be inserted betweenT2 and Ti. The cathode "l of tube T2 is biased to ground over resistorR1, and the resistor is shunted by the customary bias bypass condenserC1. P is a power supply system for the tubes of all stages and comprisesa transformer l and a full-wave rectifier 2. 3 is the customarysmoothing filter and 4 is the output transformer of the amplifier.

The amplified currents of the final or output stage T1 develop a voltageE0 across the internal impedance Z0 of the power supply system P. Thisvoltage, after reduction to a value E0 by the decoupling filter 5 findsits way over path 8, and coupling resistor Re into the plate circuit ofthe low level first amplifier tube T1. The result is a feedback voltageover coupling condenser Cc between grid 6 and cathode l of tube T2 inthe used to develop across cathode biasing resistor Hr of tube T2 avoltage equal and opposite to that developed between grid and cathode oftube T2 by the normal regeneration process. Since the bias resistor R:is in series in the circuit from grid 6 over grid leak resistor R 1,ground conductor 9, bias resistor Br, and cathode l of tube T2, the twofeedbacks will exactly cancel each other, entirely eliminating thefeedback efiect to the anode of Ti- The balancing process isaccomplished by the connection of a condenser C between feedback path 8and a point between cathode l and resistor Ra.

The equivalent circuit by which a feedback potential E0 produces avoltage e1 between grid and ground in the coupling network between tubesT1 and T2 of Fig. 1 is given in Fig. 2 (assuming the internal impedance2c of E0 is negligible compared with Re). A straight-forward calculationof voltage relations gives The equivalent circuit by which the feedbackpotential E0 produces a compensating voltage across R: is given in Fig.3. A solution of the voltage relations in this circuit yields Thetendency for the feedback E0 to affect effective.

stage T1 will be eliminated when the circuit elements satisfy therelations where R,= plate resistance R: =resistance formed by R, and Rin When Equations 1 and 3 are satisfied, then e1'=e1 and the balance iscomplete and independent of frequency.

The equations assume that the impedance Zc' is small compared with thecoupling resistance Rc of the amplifier, and also that the screen gridbypass condenser Cu: is of adequate size to return substantially all ofthe screen current directly to the cathode. In practical amplifiers,these assumptions are almost perfectly realized, and to the extent thatthis is the case, the balance is independent of frequency.

In a typical example, one might use the values oi 0.50 megohm forcoupling resistor Re, 0.5 megohm for grid leak resistor Rn, 1700 ohmsfor bias resistor R1 and 4.2 mfd. for bias bypass condenser, i. e., thesame values that would be selected on the basis of usual considerations.The required value of balancing condenser C and coupling condenser Ccare then computed by Equations 3 and 4, respectively. The resultingnumerical values are (assuming that Rp is infinite) C=4.2 mid, andCc=0.014 mfd.

It will be seen, therefore, that my novel arrangement of eliminatingregeneration caused by 'a common plate impedance is both simple and Itrequires the addition of only one new circuit element, a condenser C ofreasonable size, together with control of the proportions of certainother circuit components. This balancing arrangement will reduce theregeneration by 30 to 40 db. without difilculty, and even greaterreduction can be achieved by careful adjustment of circuit constants.

A system such'as herein described for r ducing regeneration arisingfrom. a common plate impedance is also effective in reducing the humvoltage that an incompletely filtered power-supply system will introduceinto an amplifier stage. This is so because the balancing arrangementacts to prevent voltage E existing in the powersupply system P fromproducing a potential between the grid and cathode of the tube T2. Thusthe above discussion, although carried out on the assumption that E0 isa regeneration voltage, actually applies equally well when this is a humvoltage, and the hum will be reduced to the same extent as regeneration.

What I claim is:

1. In a multi-stage audio frequency amplifier, an amplifier tube in eachstage having a cathode and an anode, a path over which both the supplyvoltage and a voltage developed by amplified currents in the final stageare fed to the anode of the tube in the first stage, a cathode biasingresistor for the tube of the second stage. and means for developingacross said resistor a voltage equal and opposite to said firstmentioned voltage to balance the latter, said means including twocondensers of predetermined value connected in series with one anotherbetween the two sides of said voltage feeding path, and a connectionfrom the common junction point of said condensers and the cathode ofsaid second stage tube.

2. In a multi-stage audio frequency amplifier, an amplifier tube in eachstage having a cathode and an anode, a power supply system for all saidtubes, a path over which a voltage developed by amplified currents inthe final stage across the internal impedance of said power system isfed back to the anode of the tube in the lowest level stage, a cathodebiasing resistor for the tube of the intermediate stage succeeding thelast mentioned tube, and means for developing across said resistor avolta e equal and opposite to said first mentioned voltage to balancethe latter, said means including two condensers of predetermined valueconnected in series with one another between the two sides of saidvoltage feeding path, and a connection from the common junction pointof'said condensers and the cathode of said intermediate stage tube.

3. In a multi-stage amplifier, an amplifier tube in each stage, a powersupply system for all said tubes, a feedback path to the anode of thetube in the first stage over which is transmitted the voltage developedby amplified currents in the final stage across the internal impedanceof said power system, a decoupling filter in said path, a connectionincluding a coupling condenser between the tube in the first stage andthe tube in the second stage, a biasing resistance for the tube in thesecond stage, and means including a condenser for developing across saidbiasing resistance a voltage equal and opposite to said first mentionedvoltage to balance the latter, said last mentioned condenser beingconnected from the positive side of said path of said power system tothe cathode end of said biasing resistance, whereby the feedback voltagereaching said anode of said first stage tube and passing through saidcoupling condenser is efiectively balanced out.

4. In a multi-stage audio frequency amplifier, an amplifier tube in eachstage having an anode, a cathode and a grid, a power supply system forall said tubes including a transformer and 9. rectifler, a path fromsaid power supply system to the anode of the tube in the first stage andover which is transmitted the voltage developed by amplified currents inthe final stage across the internal impedance of said power system, adecoupling filter and a coupling resistance in said path, a connectionincluding a coupling condenser between the anode of the tube in thefirst stage and the grid of the tube in the second stage, a connectionbetween the cathode and grid of the second tube including a grid leakand a biasing resistance, a bias bypass condenser shunted across saidbiasing resistance, and a balancing condenser connected irom theconnection between the biasing resistance and the cathode of the secondtube and the positive anode-feeding side of said feedback pathdeveloping across said biasing resistance a voltage equal and oppositeto said first mentioned voltage to balance the latter.

5. In a multi-stage amplifier, an amplifier tube in each stage having ananode, a cathode and a grid, 9. power supply system for all said tubes,a path from saidpower supply system to the anode oi the tube in thefirst stage, a connection including a coupling condenser between theanode of the tube in the first stage and the grid of the tube in thesecond stage, a connection between the cathode of the second tube andground including a biasing resistance, and a condenser connected betweenthe biasing resistance and said path developing across said biasingresistance a voltage equal and opposite to the hum voltage of the owersu pl system to balance the latter.

6. In a multi-stage amplifier, an amplifier tube in each stage, a commonpower supply system for all of said tubes, a feedback path from thepower pp y System to a lower level tube over which parasitic voltagedeveloped across said system is transmitted to the grid of said lowerlevel tube, a connection between the cathode and grid of said lowerlevel tube including a. grid leak, a biasing resistance and a bypmcondenser shunted across said biasing resistance, and a balancingcondenser connecting the cathode of said tube and the feedback path,developing across said biasing resistance a voltage equal and oppositeto said parasitic voltage to balance the latter.

